Helium leak test for spherical tanks

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In general, pressure vessel manufacturers that deal with category 1 or 2 vessels or low-temperature equipment are familiar with helium leak testing. However, helium leak testing is rarely used for large spherical tanks. During the manufacturing and installation process of such tanks, various types of weld inspections are carried out; in addition to non-destructive testing methods (TOFD, ultrasonic UT, magnetic particle MT, penetrant PT, which are used both during manufacturing and installation), hydraulic testing and airtightness testing are also employed as ways to detect weld leaks. How many non-destructive testing procedures must a spherical tank undergo to pass? Under what circumstances does a spherical tank need to undergo a helium leak test? The spherical tank that Shula was involved in testing for helium leakage was a 2000-cubic-meter vacuum spherical tank; very high standards were required regarding the vacuum level of such tanks, which in turn meant high requirements for the welds as well. That’s why a helium leakage test was conducted. For the liquid ammonia spheres I’ve come into contact with recently, the design institute also requires helium leak testing. Liquid ammonia is highly corrosive, and strict requirements are placed on welds; therefore, the design institute mandates helium leak testing. Since liquid ammonia spheres were manufactured in the previous phase, no helium leak testing was carried out. By referring to relevant standards and consulting several helium testing manufacturers, I gained an understanding of the basic principles and procedures, and also made a preliminary estimate of the commercial costs. Commercial price: Taking a 3,000-cubic-meter sphere tank as an example: The cost of helium is approximately 100,000–150,000 yuan, with the price varying depending on the purity level of the helium ； Testing cost: 120,000–150,000; total: 220,000–300,000 per unit. (Note: These are estimated figures; everyone is welcome to provide feedback on these prices.) General principles and steps: The following points should be taken into account when conducting nitrogen leak tests: 1. Nitrogen is lighter than air and can cause asphyxiation; operators must take precautions to protect themselves. 2. The testing area should be dry, well-lit, with no significant air currents or electromagnetic interference from external sources. 3. The environmental humidity in the testing area should be below 75%. 4. If vacuuming is required for the equipment being tested, the ambient temperature in the testing area should be at least 15.6°C. 5. The scale range of the pressure gauge should be multiple times the maximum test pressure; under any circumstances, its range must not be less than 1 time the maximum test pressure, nor more than several times that pressure. 6. It is necessary to ensure that all components of the equipment being tested can withstand the increased pressure, pressure retention, vacuum conditions, or heating during the testing process. 7. During testing, the valves connecting the equipment being tested to the pressure source should be closed. 8. The equipment being tested should be dry and clean, with no contaminants on the weld surfaces that could conceal leaks. The nitrogen leak detection methods are divided into three types based on the location where nitrogen is introduced: sniffing probe detection, tracer probe detection, and shroud detection. Specific testing method: 1) Internal nitrogen. The nitrogen concentration at the test pressure must be at least 10% by volume; when filling the container with nitrogen, it is necessary to ensure that nitrogen does not form layers inside the container. To achieve this, an appropriate amount of nitrogen can be used to break the vacuum in the container, after which air or nitrogen can be added to reach the desired pressure; 2) Fill the container with a well-mixed nitrogen-containing gas. A manifold is used, and nitrogen-air or nitrogen is used for pressurization; multiple ports are employed to gradually add the required amount of nitrogen. 3) Test pressure. The test pressure for the equipment shall be no more than 259% of the equipment’s design pressure, and also no more than 0.103 MPa; 4) Holding time. Before inspection, the test pressure must be maintained for at least 30 minutes. Before pressurizing with nitrogen for the first time, if the components have already been partially evacuated, nitrogen will spread immediately, so the minimum allowable holding time can be shorter than the value specified earlier; 5) Scanning distance. Use the sniffing probe tip to sweep across the surface to be inspected, keeping the distance between the probe tip and the surface at less than 665 3.2 mm during the scanning process. If a shorter distance is used in system calibration, the scanning distance during the inspection scan must not exceed that distance; 6) Scanning speed. The maximum scanning rate shall be specified during system calibration; 7) Scanning direction. The inspection scan starts from the lowest point of the system being inspected, and then proceeds upward gradually. For details, you can refer to HG/T20580-2020.